1. Field of the Invention
The present invention relates to a mounting apparatus for a releasable (e.g., shock absorbing) steering column and is more particularly concerned with such a mounting apparatus incorporating an improved arrangement for establishing an electrical connection between the steering column and the body of a vehicle.
2. Related Background Art
In the event of a collision of an automobile a so-called secondary collision takes place in which the driver collides with the steering wheel, subsequent to so-called primary collision in which the automobile collides with for example, another automobile.
In order to protect the driver by reducing any shock on the driver at the time of the secondary collision, it has been a common measure to adopt a so-called collapsible steering shaft in conjunction with a shock absorbing steering column. When a strong impact is applied to the steering shaft, it collapses along its length, and a shock-absorbing structure for the steering column absorbs the shock of impact.
The structure for mounting such a shock absorbing steering column on a stationary part of the automobile is required to release the steering column so as to allow the steering column to be forwardly displaced when a strong shock is applied thereto.
Conventionally, a structure as shown in FIGS. 8 to 10 has been used for securing a steering column to, for example, the underside of a dash board.
Referring to FIG. 8, a steering shaft 1 is adapted to be rotated by a steering wheel 2 fixed to the upper end thereof. The steering shaft 1 has a collapsible-type construction so that its length can be reduced when it is impacted. A tubular steering column 3, through which the steering shaft 1 is extended, is secured at an intermediate portion thereof to a part of the automobile body 4, e.g., to the underside of a dash board. This steering column 3 also has a collapsible construction so that it can be collapsed in the lengthwise direction in response to a shock thereby absorbing the shock.
A support bracket 5, which is formed by bending a metal sheet, is fixed by, for example, welding to the outer peripheral surface of the intermediate portion of the steering column 3. Mounting web portions 6 for securing the support bracket 5 to the body 4 are formed on both sides of the support bracket 5. U-shaped notches 7 are formed in the mounting web portions 6 in such a manner as to open at side edges of the mounting plates 6 adjacent the steering wheel 2.
A clamping plate 8, which is formed by being a metal sheet in a U-like form, is fitted on the above-mentioned side edge of each mounting web portion 6 from the same side as the steering wheel 2, i.e., from the right upper side as viewed in FIG. 8, in such a manner as to cover the notch 7. The plate 8 has a function of allowing the steering column to be released from the mounting structure with a light force at the time of a collision. More specifically, the plate 8 has a pair spaced flat plate portions 9 which are provided with apertures 10. Slip layers made of an insulating material such as polytetrafluoroethylene (PTFE) are formed on the opposing inner surfaces 9a of the flat plate portions 9 contacting the mounting web portions 6.
The steering column 3 is secured at its intermediate portion to the body through the support bracket 5 having the described construction. To this end, a bolt 11 is driven into a threaded hole 12 of the body 4 through a washer 13, the apertures 10 formed in the plate 8 and through the notch 7 formed in the mounting web portion 6. Consequently, the mounting web portion 6 is strongly clamped by the clamping plate 8, whereby the support bracket 5 is supported on the body 4.
Since the steering column 3 is supported at its intermediate portion on the body 4 in the manner described, the steering column 3 is allowed to displace forwardly (leftward as viewed in FIG. 8) to reduce the risk of injury to the driver. More particularly, when a shock is applied to the steering wheel 2 due to a secondary collision after a primary collision, the shock is immediately transmitted to the steering column 3 so that the steering column 3 is strongly forced in the axial direction. As a result, the notches 7 formed in the mounting web portions 6 separate from bolts 11 and the steering column 3 is collapsed along its length through the collapsible construction thereof (not shown).
Because the slip layers are formed on the inner surfaces 9a of the flat plate portions 9 of each clamping plate, the bolts 11 can escape from the notches 7 in the mounting web portions 6 with a comparatively small force. Thus, the counter-shock produced when the mounting structure is disengaged can be diminished.
Despite their advantage, the slip layers present a problem when electric circuits, such as a circuit for an alarm, are incorporated in the steering wheel 2. In order to supply power to such electric circuits, as by grounding for example, the structure for mounting the steering column 3 must provide an electrical connection between the steering column 3 and the vehicle body 4. However, because the slip layers are made of insulating material and the upper and lower surfaces of the supporting plate portions 6 of the support bracket 5 are in contact with the slip layers, the steering column 3 is electrically isolated from the body 4.
Therefore, an arrangement as shown in, for example, Japanese Utility Model Application Laid-open No. 61-1472 has been proposed to provide an electrical connection between the steering column 3 and the body 4.
FIGS. 11 and 12 show two types of constructions for providing electrical connection between the steering column 3 and the body 4.
Referring first to FIG. 11 showing a first arrangement, an annular conductive portion 15 is connected to an end edge of the clamping plate 8 through a narrow connecting portion 14. In a second arrangement shown in FIG. 12, an annular conductive portion 15 is connected to one end of a conductor line 16 which is connected at its other end to the plate 8.
In each of these arrangements, the conductive portion 15 has an exposed metallic portion. In addition, no friction layer is formed on the external surface 9b of the flat plate portions 9 of the clamping plate 8. It is therefore possible to complete an electrical connection between the steering column 3 and the body 4, through the support bracket 5, the conductive portion 15, the plate 8 and the bolt 11, by connecting the conductive portion 16 to the mounting web portion 6 by, for example, rivets or screws.
The known steering column mounting structure having the described constructions suffers from the following problems.
The structure employing the first arrangement shown in FIG. 11 requires a large force for disengaging the mounting portion at the time of a collision. More specifically, in order for the bolt 11 to escape from the notch 7 in the mounting web portion 6 so as to disengage the mounting structure, it is necessary to apply a force which can overcome the sum of the sliding friction force between the clamping plate 8 and the mounting web portion 6 and the force required for rupturing the connecting portion 14. These forces are illustrated in FIGS. 13A and 13B. As will be appreciated by those skilled in the art, the sliding friction force involves components of static friction and dynamic friction, the static friction being generally greater than the dynamic friction so that the sliding friction is maximized at the moment at which a relative movement is caused between two sliding surfaces. In the arrangement shown in FIG. 11, the sliding movement of the mounting portion 6 relative to the clamping plate 8 begins concurrently with the rupture of the connecting portion 14. This is because movement of clamping plate 8 relative to the body 4 and the washer 13 cannot occur before a displacement of the mounting web portion 6 relative to the clamping plate, since the friction between the outer surfaces 9b of the flat plate portions 9 of the plate 8 and the body 4 or the washer 13 is large. Therefore, in order to disengage the mounting structure at the time of a collision, an external force has to be applied which is large enough to overcome a total force shown in FIG. 13C which is the sum of the maximum sliding friction shown in FIG. 13A and the force shown in FIG. 13B for rupturing the connecting portion 14.
The increase in the force required for disengaging the mounting structure is not so extreme and may seem to present no problem. In order to ensure the safety of the driver, however, the force required for disengaging the mounting structure should be minimized hence there still remains a need for improvement.
In the arrangement shown in FIG. 12, the movement of the mounting web portion 6 relative to the clamping plate 8 begins at a moment which is different from the moment at which the end of the lead line 16 is disconnected, so that the above-mentioned problem is averted. In this case, however, inconvenient measures are required for administration of parts and assembly of the structure because of the use of three separately prepared parts 8, 15 and 16, with the result that the cost of the steering column mounting structure is increased.
In the arrangement shown in FIG. 11, it is possible to hold the clamping plate 8 at a predetermined position on the mounting web portion 6, by fixing the conductive portion 15 to the mounting plate portion 6. In the structure shown in FIG. 12, however, such an advantage cannot be achieved because of a slack of the lead line 16.